Our team is working on various efforts that all aim to expedite and promote enhanced rock weathering (ERW) as one of the most promising options of scalable climate solutions for the next few decades.

We have been working on various enhanced weathering concepts for 3 years now and we have learned a lot. Especially our Project Carbdown creates a constant stream of knowledge: we are trying to measure the speed of the weathering process on agricultural land together with the help of over 20 scientists from various institutions and universities.

While digging deeper into ERW every few months we encounter yet another rabbit hole moment when we come across another important parameter that influences the weathering activity and that needs attention.

Latest Rabbit Hole Example: It’s not the Grain size!

Most people (and we) believed that the weathering rate is always higher the smaller the basalt grains are. Well, that’s not automatically correct. When we did detailed lab analysis it turned out that our coarse basalt (“crashed sand”, grains 2.000 microns and smaller) actually has a 30%-40% larger surface area than our very fine basalt powder (<200 microns). BTW: Would you have thought that one gram of crushed basalt can have a surface area of up to 3 square meters?

The bigger grains of the crushed sand seem to have more cracks, scars, valleys, mountains while the fine stuff seems to have rather rounded grains. Very likely this will mean that the weathering rate isn’t worse for the bigger grains, it might even be higher, because the contact area between rock and water is larger.

Which is great because the fine material is hard to work with at scale in the field and we found the coarse stuff to be much easier to spread. Plus, grinding the rocks requires a lot of energy meaning that the coarse stuff carries a smaller CO₂ emissions backpack when it comes out of the mine.

The video on the right gives you an idea of how the crushed sand looks like.

Wait, there is more!

Then, quickly, the next question, the next rabbit hole showed up: What will happen to the surface area advantage of the coarse basalt over time? Will the weathering process…

• …either grind deeper into the scars and valleys of the rock, increasing the surface area even more (which would effectively mean that the weathering surface area is increased over time and speeds up the chemical reactions)

• …or will the weathering rather level valleys and mountains of the grains and decrease surface area in the process (which would effectively mean that the weathering slows down more and more).

For ERW projects this will make a huge difference in how long it takes to reach most of the chemical weathering potential of the crushed rock (which is about 1 ton of CO₂ for 3 tons of basalt).

We haven’t found a person or a study that has the answer specifically for basalt use for carbon dioxide removal on croplands (if you know one, let us know via email or send a tweet to @carbon_drawdown).